Normal bone remodeling, which occurs throughout the adult life in order to preserve the integrity of the skeleton, involves bone resorption by osteoclasts and bone formation by osteoblasts. Thus, any interference between the balance in bone formation and bone resorption can affect bone homeostasis, bone formation and repair.
The osteoblasts come from a pool of marrow stromal cells (also known as mesenchymal stem cells; MSC). These cells are present in a variety of tissues and are prevalent in bone marrow stroma. MSC are pluripotent and can differentiate into osteoblasts, chondrocytes, fibroblasts, myocytes, and adipocytes.
Osteoporosis is a major cause of morbidity and mortality in the elderly and the annual cost to the U.S. health care system is at least ten billion dollars. Both men and women suffer from osteoporotic bone loss with age. Decreases in sex hormones with age are thought to impact these detrimental changes. For example, osteoporosis increases in women after menopause.
Accumulating evidence suggests that the number and activity of osteoblastic cells decrease with age, however the reason for this change is not clear. Additionally, there is an increase in formation of adipocytes in osteoporotic bone marrow that appears to be at the expense of osteoblast formation. Moreover, the volume of adipose tissue in bone increases with age in normal subjects, and is substantially elevated in age-related osteoporosis, with the number of adipocytes adjacent to bone trabeculae increasing in parallel to the degree of trabecular bone loss. Based on this and similar observations, it has been suggested that bone loss in age-related osteoporosis is at least in part due to a shift from osteoblastic differentiation to the adipocytic pathway.
Bone fracture healing is impaired in the elderly, and others demonstrating a reduced number and activity of the MSC that would normally migrate into the fracture site and allow for new bone formation to occur.
At present, the only treatments for osteoporosis are those that target bone resorption by osteoclasts. These FDA approved therapeutics include the bisphosphonates, hormone replacement therapies, such as selective estrogen receptor modulators, calcitonin, and vitamin D/calcium supplementation. However, these treatments only result in only small improvements in bone mass, and are not sufficient for total prevention or treatment of osteoporosis.
Currently, the only FDA approved anabolic agent for the treatment of osteoporosis is parathyroid hormone (PTH). PTH is currently thought to increase bone formation by inhibiting osteoblast apoptosis. PTH has been found to increase bone mass upon intermittent injection and reduce bone fracture incidence in osteoporotic patients. However, the dose must be strictly regulated since continuous treatment with PTH and/or its accumulation may have adverse systemic effects upon the patient. Additionally, PTH treatment is quite expensive. Consequently, PTH treatment has been reserved for only the most severely osteoporotic patients.
Other potential therapeutics for enhancing bone formation by osteoblasts include sodium fluoride and growth factors that have a positive effect on bone (for example insulin-like growth factors I and II and transforming growth factor beta). However, thus far these factors have had undesirable side effects.
The use of stem cells for treating bone related disorders in humans has also been examined. For example, osteogenesis imperfecta is a skeletal disease in which the patient's osteoblasts do not make collagen I in a proper form, resulting in the brittle bones. Infusion of osteoblastic progenitor stem cells from a healthy individual into a diseased individual has been shown to improve bone density in these patients. Further, stem cells can be isolated from an individual, expanded in vitro, stimulated to become cartilage forming chondrocytes, and infused back into arthritic joints where the cartilage is remade.
Therefore, agents and methods for regulating bone homeostasis, bone formation and bone repair are desired.